Basal Body Temperature as a Guide to Cycle and Fertility

Basal body temperature (BBT) is your body's temperature at complete rest, measured first thing in the morning before you move, speak or get out of bed. It is one of the most scientifically well-established methods for understanding the female cycle, identifying ovulation and tracking fertility naturally. Measuring your BBT every day opens a window into your own biology that no other tool can match for precision.

After ovulation, progesterone levels in the blood rise. Progesterone is the dominant hormone of the second half of the cycle and it raises basal body temperature by a measurable 0.2 to 0.5°C (0.4 to 0.9°F). This rise persists until the next period begins. It is a reliable, body-generated signal that confirms one thing clearly: ovulation has occurred. No app in the world can predict this rise from statistical averages alone. But Daysy measures it every morning with a precision of 0.05°C and interprets it scientifically, giving you a clear, evidence-based picture of your cycle every single day.

Every morning, immediately after waking up and before getting out of bed, at the same time each day. Place the sensor under your tongue for 60 seconds. That's it. Daysy handles the entire analysis automatically. You measure, Daysy does the rest.

Your BBT curve tells the story of your cycle. During the first half of your cycle, the follicular phase, temperature sits at a lower baseline level. After ovulation it rises noticeably and stays elevated until your next period begins. This biphasic pattern is the foundation of the BBT method. Learning to read your own curve means being able to see ovulation, cycle length, luteal phase duration and meaningful health changes – all reflected in a simple daily temperature reading.

Your BBT curve provides far more than just information about fertility and ovulation. It can also offer clues about hormonal imbalances, a luteal phase that is too short, anovulatory cycles or other health-related patterns worth paying attention to. Women who observe their curve over several months often recognize patterns that become genuinely valuable in conversations with their OB-GYN, midwife or healthcare provider – giving them real data to bring to those appointments rather than just a general sense that something feels off.